1. Field of the Invention
The present invention relates to superconducting wires and cables, and particularly to a superconducting wire and cable excellent in stability and effective in the production of superconducting appliances adapted for a high electric current density.
2. Description of the Prior Art
FIG. 1 is a schematic cross-sectional view of the structure of a conventional superconducting wire. The superconducting wire 1 has a structure comprising a large number of superconducting filaments 3 made of NbTi, Nb.sub.3 Sn or the like and embedded in a metallic matrix 2 made of copper, a copper-tin alloy or the like. This superconducting wire 1 has the exposed surface of the metallic matrix 2.
FIG. 2 is a schematic cross-sectional view of the structure of another conventional superconducting wire. This superconducting wire 4 has a structure further comprising an insulator film 5 made of an inorganic or organic substance and covering the surface of the same wire element 1 with a large number of filaments as shown in FIG. 1.
In general, as shown in FIG. 3, a number of superconducting wires 1 shown in FIG. 1 or superconducting wires 4 shown in FIG. 2 are assembled into a cable 6, with which a superconducting appliance such as a superconducting magnet is produced. Flat cables produced by assembling a number of superconducting wires or cables as shown in FIG. 3 are also often used. An indispensable requirement for operating a superconducting appliance stably is to suppress the transient heat during excitation thereof as much as possible, which heat is caused by movement of a superconducting cable or the like. Accordingly, there have been proposed various ideas including application of high winding tension in the course of production of a coil for a superconducting magnet, sufficient precompression of a whole coil after the completion of winding the coil, and equalization of the heat shrinkages of individual members constituting a magnet.
Meanwhile, the instability of superconducting cables has heretofore been often observed. This is such a phenomenon that the superconducting state of wires is quenched through heating-up thereof caused by frictional heat generated, for example, by minute movement of the whole cable and/or wire elements constituting the cable and/or cracking of an epoxy resin used for the fixation of the superconducting wires, which is attributed to a strong electromagnetic force created when a large electric current is flowed through the superconducting cable. As described above, conventional superconducting wires and cable are substantially in a defenceless state against the transient heat, which may occur on the surfaces thereof, without any satisfactory countermeasures thereagainst with a view to providing sufficient stability for those wires and cables. The above-mentioned phenomenon tends to more strongly manifest as an operating current density is increased more. Such frictional heat generation entails an extreme difficulty in producing superconducting appliances capable of sufficiently exhibiting a feature of high current density inherent in superconductors.
The reason why the superconducting state of cables is quenched by the movement of the superconducting cables and/or wire elements constituting the cables is that heat generated on the surfaces of the superconducting wires 1 or 4 as the wire elements through friction between the cables or wire elements reaches superconducting filaments 3 inside the superconducting wires 1 or 4 to heat up the filaments 3 beyond the critical temperature thereof. As shown in FIGS. 1 and 2, however, conventional superconducting wires have a metallic matrix 2 either exposed or covered with an insulator film 5. Thus, either type of conventional superconducting wires are of such a structure that most of heat generated on the surfaces of the superconducting wires 1 or 4 reaches the superconducting filaments 3 inside the wires 1 or 4.
More specifically, where the metallic matrix 2 is exposed as shown in FIG. 1, heat generated on part of the surface of the superconducting wire 1, though partly conducted in the circumferential and longitudinal directions of the wire 1, mostly reaches the superconducting filaments 3 in a very short time because the superconducting filaments 3 is very close to the surface of the wire 1.
Also where the surface of the metallic matrix 2 is covered with the insulating film 5 as shown in FIG. 2, most of heat generated on part of the surface of the superconducting wire 4 reaches the superconducting filaments 3 despite the relatively low rate, or speed, of heat conduction from the surface of the wire 4 to the superconducting filaments 3 due to the barrier effect of the insulator film 5 on heat conduction because the heat is hardly conducted in the circumferential and longitudinal directions of the insulator film 5, or the wires 4, as well due to the extremely low thermal conductivity of the insulator film 5.
As described above, the conventional superconducting wires involve a problem of the superconducting state thereof being quenched with a high probability once heat is generated on the surfaces thereof through movement thereof or the like. This problem has been bottleneck in the production of highly reliable superconducting appliances.